We've had success encouraging Lightning adoption by:

1) Administering all trainings, demos, and reference-work in Lightning so that it's the UI users are most familiar with.

2) On quick-fixes, tweaks, or changes that the user should feel empowered to do on their own, making the adjustments on their comp, in their org, in Lightning so that they can see the workflow and feel confident enough to do it solo next time.

3) Use "classic" rarely, and only in cases where we're doing something that appears "complicated" to encourage the dichotomy between Lightning: easy, user-friendly, and Classic: complex, ugly, guru-level ;P"

#1 has the most impact, but the other 2 also help supplement the effect and we've had no troubles getting ##LightningStrikes adoption up!

Happy New Year!